Magnetic recording medium

ABSTRACT

The present invention provides a magnetic recording media made possible to improve reliability and durability of the magnetic recording media with an extremely thin overcoat. A magnetic recording media made by forming at least a magnetic coat and an overcoat on a substrate of non-magnetic magnetic disk on which surface a liquid lubricant having a structure of perfluoropolyether is coated, wherein average roughness of overcoat surface Ra is less than 0.8 nm, said overcoat being a diamond-like-carbon with a thickness in a range of 1.5-4.5 nm, and said lubricant on said carbon overcoat containing a lubricant component having a structure represented by chemical formula (1):  
                 
 
     (where, p=5-36, q=4-30, x=1-5)

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates to a magnetic recording media made byforming a under layer, a magnetic layer and an overcoat on a substrateof a non-magnetic magnetic disk, over which a liquid lubricant having astructure of perfluoropolyether is coated, particularly to a magneticdisk media having a very thin overcoat of several nano meters.

[0003] 2. Description of the Related Art

[0004] Recording density of the magnetic disk devices is remarkablyincreasing and even a density of more than 10 G bits per square incheshas been announced recently. In order to achieve such a high recordingdensity, it is mandatory to bring the space between magnetic head andmagnetic recording layer of the magnetic disk as close as possible, tothe extent of less than 20 nm at present level.

[0005] Most of this space is occupied by overcoat thickness on themagnetic recording layer on the magnetic disk and flying height ofmagnetic head. Therefore, overcoat thickness and flying height of themagnetic head are in a trade-off relationship. For a magnetic disk, anovercoat thickness as thin as possible, but with a high wear resistanceshould the magnetic head touch the overcoat, is required. Judging fromthe current space between magnetic head and magnetic recording layer ofmagnetic disk, the future task is to achieve a thickness of less than 5nm.

[0006] In the recent magnetic disk devices, load-and-unload (L/UL)method is more and more employed than the conventionalcontact-start-stop (CSS) method. In the L/UL method, magnetic head isunloaded parting from the magnetic disk when the disk is stopped, andwhen the disk starts to rotate, the magnetic head is loaded on the disk.On the other hand, in the conventional CSS method, the magnetic headstays in contact with the disk, and when the disk starts to rotate,magnetic head flys up by air flow caused by rotation. However, in theL/UL method, although requirement for wear resistance is mitigated tosome extent, the disk should withstand to a shock when loading on, and asudden contact by a mal positioning of the magnetic head which canhappen even in normal operation.

[0007] With respect to overcoat, as disclosed in the U.S. Pat. No. Re.32464, carbon based materials have been used heretofore. Many methodsfor enhancing hardness of the carbon based overcoat to make it thinnerhave been proposed. For example, a method of adding hydrogen, asdisclosed in the Japanese Patent Laid-Open S 59-154641, and a method ofadding nitrogen in a sputtering process, as disclosed in the JapanesePatent Laid-Open H 8-106629, to increase hardness of carbon basedovercoat and make it thinner.

[0008] Attention has been riveted since many years todiamond-like-carbon (DLC) which has especially high hardness among thecarbon based overcoats. Many methods for preparation of DLC have beenproposed such as Japanese Patent Laid-Open S59-154641, as cited before,describing a chemical vapor deposition method (heretofore abbreviated asCVD method) in which carbon is deposited on a substrate by decomposinghydrocarbon gas by electric discharge, or ion beam deposition method(heretofore abbreviated as IBD method) in which a hydrocarbon is ionizedby irradiation of thermoion which is generated by a heated filament, andcarbon is deposited on a substrate by collision of the said ion beamaccelerated by bias voltage applied to the substrate.

[0009] Particularly the IBD method is easy to form an overcoat having ahigh hardness, as described in the 24^(th) column of the Japanese PatentLaid-Open 2000-105916, teaching that “by using a thermal filament,plasma can be generated under much lower pressure than in conventionalCVD method, and as monomer molecules are less concentrated,decomposition of the monomer is more complete.”

[0010] On the other hand, as described in the Japanese Patent Laid-OpenS61-26827, waer resistance of the carbon based overcoat is not improvedmerely by increasing its hardness, but it is necessary to use fluorinebased lubricant such as those having a perfluoropolyether structure atthe same time. Thus, optimization of combination with lubricant is animportant factor in designing magnetic disk.

[0011] Recently, to improve performance of the lubricant, mixing anadditive having cyclic phosphazene, as described in the U.S. Pat. No.5908817, and a lubricant having cyclic phosphazene at the end ofperfluoropolyether, as described in the Japanese Patent Laid-OpenH6-220077, are proposed. Moreover, a technology to add a cyclicphosphazene to ohe end of perfluoropolyether as shown in the chemicalformula (1) was developed recently by Matsumura Oil Research Corp., (theJapanese Patent Application H11-267696).

[0012] With respect to the technology of mixing an additive having acyclic phosphazene structure with perfluoropolyether lubricant isdisclosed in the Japanese Patent Laid-Open H9-305961 and the JapanesePatent Laid-Open H10-251676. Example of the prior art shows that, byforming the above mentioned film on DLC overcoat, reduction of frictionforce, improvement of durability, reduction in contamination of the headand suppression of lubricant decomposition are achieved.

[0013] Furthermore, it is known, as described in the Japanese PatentLaid-Open H11-328647, that material for base-coat of magnetic recordinglayer also affects wear resistance of magnetic disk.

SUMMARY OF THE INVENTION

[0014] Comparing to the nitrogen-added carbon overcoat, in case of thediamond-like-carbon (DLC), as described above, although the filmhardness is high ,easiness of heat generation in the magnetic diskdevice and lubricant spin-off due to air shear force by rotation of thedisk have been the problems. Therefore, it is necessary to find out notonly the kind of overcoat material, but also an optimal combination withlubricant in order to achieve enough wear resistance with overcoatthickness of less than 5 nm.

[0015] On the other hand, the less the roughness of surface of magneticdisk is, the closer the magnetic head can approach to the disk,resulting in an advantage of increasing recording density. However, incase a sudden contact of the magnetic head happens, more damage could begiven to the overcoat because of a large friction due to a large contactsurface area between the magnetic head and the magnetic disk. Therefore,the proper surface roughness should be selected.

[0016] Furthermore, in case overcoat has a large Young's modulus,deformation of the overcoat is small leading to a smaller contact areafor the head, and a higher contact pressure resulting therefrom tends topromote waer. Also from this point of view, it is important to find outan optimal combination of overcoat material, lubricant and roughness ofthe surface.

[0017] As shown in the Japanese Patent Laid-Open H9-305961 and theJapanese Patent Laid-Open H10-251676, a lubricant coat made by mixing anadditive having cyclic phosphazene structure tends to causecontamination of the head, leading to a larger friction force. Thiscontamination is caused by phase separation of the additive which sticksto the head. As the phase separation of additive depends on the overcoatmaterial, ana an increase in friction force depends on surface shape ofthe magnetic disk, it is necessary to find out an optimal combination oflubricant, overcoat and surface shape.

[0018] The inventors of the present invention, after studied variouscombination of overcoat, overcoat thickness, lubricant and surfaceroughness, has found out that by using a diamond-like-carbon (DLC)prepared by IBD method and lubricant as defined by the chemical formula(1), and making surface roughness (Ra) of magnetic disk, i.e. averagesurface roughness of the overcoat less than 0.8 nm, an enough durabilityis achieved even with overcoat thickness of less than 5 nm, i.e. from1.5 to 4.5 nm, and that above achievement is not realized if any one ofthe diamond-like-carbon, lubricant of the chemical formula (1) or Ralacks.

[0019] Where the upper limit of Ra was defined because it was found thata larger Ra causes rather reduction of wear resistance.

[0020] Namely, in the magnetic recording media made by forming anunderlayer, a magnetic layer and an overcoat on a non-magnetic magneticdisk, on which surface a liquid lubricant having a perfluoropolyetherstructure is coated, the feature of this invention is in a magneticrecording media where the said overcoat has an average roughness, Ra, ofits surface less than 0.8 nm, made of diamond-like carbon, having athickness within the range of 1.5-4.5 nm, and the lubricant layer on theovercoat contains lubricant component having the,structure defined bychemical formula (1).

[0021] (where p=5-36, q=4-30 and x=1- 5)

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1 is a figure showing change in friction force with change insurface roughness of substrate.

[0023]FIG. 2 illustrates effectiveness of this invention in a region ofan ultra thin overcoat.

[0024]FIG. 3 is a figure showing relationship between lubricantconcentration and head contamination.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0025] In the present invention, a glass substrate available in themarket for magnetic disk is suitably used as a substrate fornon-magnetic magnetic disk. In order to reflect difference in surfaceroughness of the magnetic disk, those having Ra of 0.35-0.8 nm, and Rp4.0-7.0 nm were prepared.

[0026] For the magnetic layer, Co alloy is generally used. Under themagnetic layer, an underlayer made of Cr alloy, and a seed layer made byCo alloy or Ni alloy etc.

[0027] An overcoat mainly composed of carbon is formed on a magneticlayer. For the overcoat, a diamond-like carbon formed by IBD method issuitable. Thickness of the overcoat was within a range of 1.5 nm -4.5nm. in this case, lubricant can be a mixture of a lubricant having astructure defined by chemical formula (1) and Fombrin Z-DOL™, as long asthe lubricant of the chemical formula (1) is contained more than 30%.Furthermore, it is preferred that the main chain of the lubricant of thechemical formula (1) has an average molecular weight of 1,500-2,500.

[0028] The present invention is described in more detail by thefollowing embodiments. Reference and test samples were prepared as shownin Table 1, with kind of substrate, kind of overcoat, thickness ofovercoat, kind of lubricant, thickness of lubricant and heat treatmenttemperature as parameters. TABLE 1 Embodiment and comparative example ofthe present invention Sample Kind of Kind of Overcoat Heat Lubricant No.substrate overcoat thickness Lubricant treatment thickness Note ASubstrate 1 Overcoat 4 4.0 nm Z-DOL ™ 80° C. 1.5 nm Comparison BSubstrate 1 Overcoat 4 4.0 nm Chemical formula (1) 80° C. 1.5 nmEmbodiment C Substrate 2 Overcoat 1 4.0 nm Chemical formula (1) 80° C.1.5 nm Comparison D Substrate 2 Overcoat 2 4.0 nm Chemical formula (1)80° C. 1.5 nm Embodiment E Substrate 2 Overcoat 3 4.0 nm Chemicalformula (1) 80° C. 1.5 nm Embodiment F Substrate 2 Overcoat 4 1.0-5.0 nmChemical formula (1) 80° C. 1.5 nm Embodiment G Substrate 2 Overcoat 11.0-5.0 nm Z-DOL ™ 80° C. 1.5 nm Comparison H Substrate 2 Overcoat 4 4.0nm Chemical formula (1) 80-120° C. 1.5 nm Embodiment I Substrate 3Overcoat 4 4.0 nm Z-DOL ™ 80° C. 1.5 nm Comparison J Substrate 3Overcoat 4 4.0 nm Chemical formula (1) 80° C. 1.5 nm Comparison

[0029] A glass substrate of 65 mm diameter was purchased and used forEmbodiment shown in Table 1. Three substrates with different roughnesswere used. Substrate 1 (Ra 0.37 nm, Rp 5.45 nm) and Substrate 2 (Ra 0.71nm, Rp 5.58 nm) illustrate the embodiment of the present invention,while Substrate 3 (Ra 0.87 nm, Rp 7.38 nm) has a higher roughness forcomparative example. Kind and measured surface shape of the substratesare shown in Table 2. TABLE 2 Kind of the substrates Roughness ofsubstrate Sample Ra, nm Rp, nm Substrate 1 0.37 5.45 Substrate 2 0.715.58 Substrate 3 0.87 7.38

[0030] Roughness of substrate was measured by AFM (Scanning probemicroscope Nanoscope III™ manufactured by Digital Instruments) intapping mode with scanning size of 10×10 μm, scan rate of 1 Hz, numberof samples 512, Z-limit 440V, filter treatment Flatten, cantilever madeof single crystal silicone, and tip curvature radius of 5-20 nm. Averageroughness, Ra, and maximum profile peak height, Rp, are automaticallycalculated by this AFM device. Measurement were made several times andthe average value was employed. Scattering of the values were 0.1 nm foraverage roughness, Ra, and about 1.5 nm for maximum profile peak height,Rp.

[0031] Coating was made on a substrate after washing, using MDP-250™(manufactured by Intevac) First, the substrate was heated to 230° C.,and then coated with a 40 nm layer of NiCrZr alloy, over which 10 nmCoCrZr layer was coated. Those layers are called seed layer forcontrolling crystal formation in undercoat. Further on this, 25 nm ofCrTi alloy layer was formed as undercoat, on which CoCrPt alloy layerwas formed as magnetic layer.

[0032] Two different overcoats were formed for comparison. One was acarbon coat formed by sputtering method, and another was a diamond-likecarbon coat formed by IBD method. Coating equipment manufactured byIntevac was used for both cases, attached to the above mentionedMDP-250™ chamber. As coating condition for sputtering method, coatingrate of 0.5 nm/s and sputtering gas composed of 18% nitrogen in argonwere employed. On the other hand, for coating by IBD method, coatingrate of 1.0 nm/s, emission current of 0.5A, bias voltage difference of200V and ethylene gas concentration of 55% (Overcoat 2), 61% (Overcoat3) and 71% (Overcoat 4) were employed. In order to investigatedependence of the overcoat on thickness, G and F in Table 1 representsovercoat formed with thickness of 1.0, 2.0, 3.0, 4.0, 4.5, 5.0 nmrespectively. Thickness of overcoat was measured quantitatively by X rayreflection method after coating 5 nm Cr on the overcoat to improveaccuracy of the measurement. SLX2000™ manufactured by Rigaku Denki wasused with X ray of CuK α1 for the quantitative measurement of thethickness. The principle of the measurement is described in the Journalof Applied Physics 66 (4) p1861, Aug. 15 1989.

[0033] The surface roughness after forming overcoat was approximatelythe same as that of the measured value for the substrate (within ±10%).

[0034] Moreover, as hardness and Young's modulus of magnetic recordingmedia depend on the kind of overcoat and the conditions of coatingprocess, hardness and Young's modulus of magnetic recording media weremeasured by the following method: NanoIndenter™ (manufactured by MTS)was used as a measuring equipment to determine hardness and Young'smodulus by CSM mode (Continuous Stiffness Mode) at 10 nm penetration.Hardness and modulus of elasticity at overcoat thickness of 4 nm wereshown in Table 3. TABLE 3 Variation of hardness and Young's modulusaccording to kind of overcoat Kind of overcoat (thickness 4 nm) Modulusof Other Hardness direct Sample Process condition (Gpa) elasticity (Gpa)Overcoat 1 Sputtering 10.5 132 Overcoat 2 IBD Ethylene 55% 14.5 165Overcoat 3 IBD Ethylene 61% 13.1 158 Overcoat 4 IBD Ethylene 71% 13.4150

[0035] Two different kinds of lubricant were applied. For comparativeexample, a lubricant with average molecular weight of 3,000 which wasmade by graduating Fombrin Z-DOL™ produced by Ausimout was applied. Inone embodiment of the present invention, a lubricant having a structureof chemical formula (1) was synthesized from a lubricant with averagemolecular weight of 2,000 made by graduating Fombrin Z-DOL™. However,this lubricant is not 100% lubricant of formula (1), but, assuming thatfunctional end group of formula (1) being modified at both ends of PFPEbased on the NMR (Nuclear Magnetic Resonance) analysis for hydrogen andfluorine, about 30% is represented by the chemical formula (1), whileremaining 70% is Fombrin Z-DOL™. Assuming that the raw material FombrinZ-DOL™ has some molecular weight distribution and the same distributionstill remained after synthesis, p and q in the chemical formula (1) wasestimated approximately as p=5-36, q=4-30. X in the chemical formula (1)is considered to be in a range of X=1-5, but this was not confirmed. Forrespective lubricant, lubricant coat was formed by dipping the magneticdisk media in a lubricant solution of various concentration usingSumitomo 3M's HFE7100™ as solvent, and pulling out the magnetic diskmedia therefrom. Then the magnetic disk media was heat treated attemperatures 80° C., 100° C., 120° C. for 30 minutes. Lubricant coatthickness was measured by FTIR (Fourier Transform InfraredSpectroscopy), and Fombrin Z-DOL™ was taken as equivalent to 0.5, 1.0,1.5 nm coat thickness.

[0036] Furthermore, a magnetic disk for detecting phase separation oflubricant and contamination of head was prepared as follows: Lubricanthaving a structure represented by chemical formula (1) was applied inExample H with various ratio of the lubricant to Fombrin Z-DO™.Concentration of the lubricant was varied as 10, 15, 20, 50, 75 and100%, and the lubricant coat thickness was adjusted to be 1.5 nm. As acomparative example, cyclic tri-phosphazene additive was added inExample H with various weight ratio.

[0037] Reliability and durability of those samples were evaluated by thefollowing way; for evaluation of wear resistance at ultra low flyingcondition, the head was placed always in contact with magnetic diskmedia by reverse rotation of motor, and the time needed for crushing wasmeasured while seeking the magnetic recording media between 15-31 mmradius. The environment temperature was 65° C. and the rotation speedwas 4,000 min⁻¹. Furthermore, in order to evaluate the vibration whenthe magnetic head touched the magnetic disk, contact friction force wasmeasured by a friction sensor using a strain gauge, at flying height ofthe magnetic head of 4 nm and rotating speed of 4,000 min⁻¹ underreduced pressure in the test facility. Test results in the followingexamples illustrate the advantage of the present invention.

[0038] First, friction force measurement was made with samples A, B, C,F, G, I and J. The results are shown in FIG. 1. Overcoat thickness wasset as 4 nm for all the samples. Lubricant coat thickness was 1.5 nm andthe temperature for heat treatment was 80° C.

[0039] As clearly shown in the FIG. 1, friction force tends to increasewith decreasing roughness of the substrate. Moreover, the friction forceappears significantly reduced in the case of combination of overcoat 4and the lubricant having structure represented by chemical formula (1).Namely, even with a low surface roughness of magnetic disk media(roughness equivalent to the substrate 1, 2), friction force is madedramatically lower in a combination of overcoat 4 and the lubricanthaving a structure represented by chemical formula (1) (Samples B, F)than those shown in the comparative examples (Samples A, C, G).

[0040]FIG. 2 shows results of wear resistance test with sample F and G.In contrast to the result in comparative example G, where the time untilcrush is extremely short, there was no crush in example F, whichillustrates an improvement to more than 10 times higher durability.

[0041] From this result it was shown that, by the method as shown in theembodiment of the present invention cited herein, magnetic recordingmedia with an extremely thin coat of several nm, which has beenheretofore not reliable due to an easy crushing, becomes possible to begiven an equivalent or more reliability than those conventional andpractical magnetic recording media with overcoat thickness more than 5nm.

[0042] Results of evaluation of wear resistance with Samples A-J withlubricant coat thickness of 1.5 nm according to embodiment andcomparative example are shown in Table 4. TABLE 4 Difference in wearresistance of each embodiment and comparative example Overcoat SampleNo. thickness Hours to crash, h A 4.0 nm 8 6 B 4.0 nm >100 >100 C 4.0 nm16 11 D 4.0 nm >100 >100 E 4.0 nm >100 >100 F 4.0 nm >100 >100 G 4.0 nm16 11 H-1 (80° C. treatment) 4.0 nm >100 >100 H-2 (100° C. treatment)4.0 nm >100 >100 H-3 (120° C. treatment) 4.0 nm >100 >100 I 4.0 nm 3 6 J4.0 nm 10 8

[0043] Paying attention to the surface roughness of the substrate,comparison was made between Sample J with substrate 3 and Samples D, E,F with substrate 1, with a result that a good wear resistance isrealized even in a case of small roughness. Moreover, paying attentionto the kind of materials of overcoat, comparison between Samples D, E, Fwith overcoat 2, 3, 4 and Sample C with overcoat 1 shows that abrasionresistance significantly improves when the overcoat is formed by IBDmethod. Sample H has a good abrasion resistance independent of healtreatment temperature. As the lubricant having a structure representedby chemical formula (1) has better adsorption to carbon overcoat thanconventional Fombrin Z-DOL™, proportion of the lubricant havingstructure represented by chemical formula (1) is supposed to increase byincreasing the temperature of heat treatment.

[0044] Phase separation of lubricant was investigated by observingsurface of the magnetic disk using optical microscopy for the exampleswith different dosage of the lubricant having a structure represented bychemical formula (1). The results are shown in Table 5. TABLE 5Occurrence rate of coagulation of lubricant Additive concentration byweight (%) Additive lubricant 10% 20% 50% 100% Comparative example ◯ X XX Embodiment ◯ ◯ ◯ ◯

[0045] In the comparative example phase separation took place, while inthe embodiment of the present invention with even 100% of the lubricanthaving a structure represented by chemical formula (1), there was nophase separation.

[0046] Effect of lubricant dosage on head contamination was alsoinvestigated. A comparison was made by a numerical system of rating forcontamination of head, after seeking the face of magnetic disk frominner to outer periphery by magnetic head for one hour. The results areshown in FIG. 3.

[0047] In a combination of an overcoat formed by CVD using ion beam andthe lubricant having a structure represented by chemical formula (1),head contamination decreased with increasing dosage of the lubricant.

[0048] From above embodiment of the present invention, it is consideredthat the magnetic disk media with surface roughness equivalent to thatof the substrates 1 and 2, with overcoat of diamond-like-carbon formedby using ion beam which is equivalent to that applied in the overcoat 2,overcoat 3, and overcoat 4, with an overcoat thickness of less than 5nm, namely, 1.5-4.5 nm, and containing lubricant having a structurerepresented by chemical formula (1), has wear resistance equal to ormore than conventional magnetic disk in spite of the extremely thinovercoat thickness.

[0049] Furthermore, scattering of lubricant during rotation of themagnetic disk media was tested by comparing thickness of lubricant coatbefore and after 12,000 min.- 1,400 hours at environment temperature of60° C. As a result, in Sample G, 43% of the lubricant coat remained,while in Sample F, 60% remained. Namely, a combination of overcoatformed by ion beam and lubricant having a structure represented bychemical formula (1) showed minimal scattering.

[0050] Thus, by the present invention, it has become possible to providea highly reliable, durable magnetic recording media.

[0051] The present invention provides a magnetic recording media with anextremely thin overcoat which has superior wear resistance and highreliability equivalent to or more than conventional magnetic recordingmedia.

What is claimed is
 1. A magnetic recording media made by forming atleast a magnetic layer and an overcoat on a substrate of non-magneticdisk on which surface a liquid lubricant of a perfluoropolyetherstructure is coated, wherein surface of said overcoat having less than0.8 nm average roughness Ra, said overcoat being a layer ofdiamond-like-carbon with less than 5 nm thickness, and said lubricantcoat on said overcoat containing a lubricant component having astructure represented by following chemical formula (1):

(where, p=5-36, q=4-30, x=1-5)
 2. The magnetic recording media accordingto claim 1, wherein said lubricant coat on said overcoat contains morethan 30% of a lubricant component having a structure represented bychemical formula (1).
 3. The magnetic recording media according to claim1, wherein principal chain of said lubricant component having astructure represented by chemical formula (1) has an average molecularweight of 1500-2500.
 4. A magnetic recording media made by forming atleast a magnetic layer and an overcoat on a substrate of non-magneticmagnetic disk on which surface a liquid lubricant of aperfluoropolyether structure is coated, wherein surface of said overcoathaving less than 0.8 nm average roughness Ra, said overcoat being alayer of diamond-like-carbon with a thickness in a range of 1.5-4.5 nm,and said lubricant coat on said carbon overcoat containing a lubricantcomponent having a structure represented by following chemical formula(1):

(where p=5-36, q=4-30, x=1-5)
 5. The magnetic recording media accordingto claim 4, wherein said lubricant coat on said carbon overcoat containsmore than 30% of lubricant component having a structure represented bychemical formula (1).
 6. The magnetic recording media according to claim4, wherein the principal chain of said lubricant having a structurerepresented by chemical formula (1) has an average molecular weight of1500-2500.